Laboratory studies have shown that the blood pH during hypothermia profoundly affects the heart and other organs. One of two pH schemes is commonly used to manage blood pH during hypothermia. The alpha-stat scheme is the method by which ectotherms (or cold- blooded animals) manage pH. In this strategy, the pH of blood is increased 0.015 pH units for every one degree decrease in temperature. When the pH is kept constant during hypothermia (7.4 in man), the pH-stat strategy is employed. This is the strategy employed by hibernators during hypothermia. Comparative physiologists have found that the intracellular pH of all organs in the ectotherm parallels the blood pH. This is different from the intracellular pH in hibernators where most organs, such as the brain and skeletal muscle, maintain a constant pH, while essential organs such as the heart and liver have an intracellular pH that follows the ectothermic scheme. The intracellular pH is important to the function of most organ systems. The hypothesis to be tested in the present investigation was that brain and heart tissue pH in heterothermic animals during hypothermia is determined by blood pH. The experimental model consisted of anesthetized sheep wrapped in cooling blankets with a radiofrequency coil placed over either the heart or skull. 31P NMR spectroscopy was performed to assess the intracellular pH at 37 degrees C and during cooling to 26 degrees C. The results show that the intracellular pH of the brain and the heart at 37oC was approximately 7.10. During hypothermia the intracellular pH of both of these tissues parallels the blood pH. That is, during pH stat management, the intracellular pH is constant, and during alpha-stat management, the intracellular pH increases with hypothermia. This has important implications for organ function during hypothermic cardiac surgical procedures in man.